A light emitting diode is receiving attention as a light source for future lighting and widely used as a light source in various fields at present due to a long lifespan, small power consumption, and environment-friendliness in comparison with lighting fixtures such as conventional fluorescent lamp, incandescent lamp, and so on. In particular, since the nitride-based light emitting diode having a large bandgap has an advantage capable of emitting light of a region from green to blue and a region of near ultraviolet rays, application fields such as LCDs and mobile phone backlights, lightings for automobiles, traffic lights, general lightings, and so on, are being widely enlarged. However, performance of the nitride-based light emitting diode is not being sufficiently improved to satisfy such needs.
The performance of the light emitting diode is generally determined based on internal quantum efficiency according to how many photons are generated by injected (injection) electrons and light extraction efficiency according to how many photons can be emitted to an outside of a light emitting diode device.
In recent times, while the internal photon efficiency of the nitride-based light emitting diode has been largely improved due to development of the epitaxial growth technique, the light extraction efficiency is very low in comparison with the internal photon efficiency. When light generated in a multi quantum well (MQW) region, which is an active layer (a light emitting layer) of the light emitting diode, is emitted, total reflection is generated at a boundary between the light emitting diode device, external air, and an external sealing material such as epoxy, sapphire substrate, or the like. Since GaN has a refractive index of about 2.5, which is relatively larger than those of air (nair=1), epoxy (nepoxy=1.5), and sapphire (nsap.=1.77), critical angle regions in which the light generated in the MQW can exit to the outside of the device are θGaN/air=23°, θGaN/epoxy=37°, and θGaN/sap.=45°, which are very limited. Accordingly, the light departing from the critical angle range and entering in a direction toward the outside of the device cannot advance to the outside but is totally reflected until the light is absorbed in the device, and thus, the light extraction efficiency is merely several percent (%), which is very low. In addition, this causes problems leading to generation of heat in/of the device.
In order to overcome the limitation of the nitride-based light emitting diode, a research has been attempted for effectively reducing total reflection through diffused reflection of light by inserting a pattern into a p-GaN layer or a transparent electrode layer of a surface of the device. In particular, it is known that, when a sub-micron photonic crystal pattern in which patterns having a uniform size are regularly and densely arranged is introduced in a light emitting diode manufacturing process, the light extraction efficiency is largely increased. However, in consideration of formation of a p-type electrode after patterning, a manufacturing process of the light emitting diode such as a packaging process or the like and production yield, it is difficult to actually commercialize the patterning process of the p-GaN layer and the transparent electrode layer. Alternatively, when the epitaxial layer is grown on a patterned sapphire substrate (PSS), similarly, the light extraction efficiency may be effectively improved due to the diffused reflection effect of the light. In the case of the PSS, essentially, the technique developed to reduce a treading dislocation density due to lattice mismatch between the sapphire substrate and the GaN epitaxial layer and increase the internal photon efficiency, or the light extraction efficiency may also be significantly improved, and may be applied to the manufacturing process of the light emitting diode. Currently, in manufacturing companies of the light emitting diodes in domestic and foreign countries, products to which the PSS are applied are in a mass production stage.